A research program is being conducted to establish methods and procedures for detecting and quantitating left-to-right, right-to-left and bidirectional shunts in neonates. Mathematical methods and computer programs have been developed for obtaining this information from thermodilution curves. The capability of these methods for making accurate measurements has been demonstrated in the cardiac catheterization laboratory using data from children with congenital heart defects, and further tests are being conducted in newborn lambs. After extensive experience has been obtained with this method and clinically suitable equipment is developed, the system will be applied, using on-line analysis of the curves by a digital computer, to monitor shunt fractions and cardiac output in neonatal patients.